Multi-piece solid golf balls having a core, cover with a casing layer disposed there between are popular today in the golf industry. The core is made commonly of a rubber material such as natural and synthetic rubbers, styrene butadiene, polybutadiene, cis-polyisoprene, or trans-polyisoprene. Often, the casing layer is made of an olefin-based ionomer resin that imparts hardness to the ball. These ionomer acid copolymers contain inter-chain ionic bonding, and are generally made of an α-olefin such as ethylene and a vinyl comonomer having an acid group such as methacrylic, acrylic acid, or maleic acid. Metal ions such as sodium, lithium, zinc, and magnesium are used to neutralize the acid groups in the copolymer. Commercially available olefin-based ionomer resins are used in different industries and include numerous resins sold under the trademarks, Surlyn® (available from DuPont), Iotek® (available from ExxonMobil), Amplify IO® (available from Dow Chemical) and Clarix® (available from A. Schulman). Olefin-based ionomer resins are available in various grades and identified based on the type of base resin, molecular weight, and type of metal ion, amount of acid, degree of neutralization, additives, and other properties. The cover of conventional golf balls are made from a variety of materials including olefin-based ionomers, polyamides, polyesters, and thermoplastic and thermoset polyurethane and polyurea elastomers.
In recent years, there has been high interest in using thermoset, castable polyurethanes (and polyureas) to make cores, casings (or intermediate layer), and/or cover layers. Basically, polyurethane compositions contain urethane linkages formed by reacting an isocyanate group (—N═C═O) with a hydroxyl group (OH). Polyurethanes are produced by the reaction of a multi-functional isocyanate with a polyol, optionally in the presence of a catalyst and other additives. The chain length of the polyurethane prepolymer is extended by reacting it with a hydroxyl-terminated curing agent.
Polyurea compositions, which are distinct from the above-described polyurethanes, also can be formed. In general, polyurea compositions contain urea linkages formed by reacting an isocyanate group (—N═C═O) with an amine group (NH or NH2). The chain length of the polyurea prepolymer is extended by reacting the prepolymer with an amine curing agent. Hybrid compositions containing urethane and urea linkages also may be produced. For example, a polyurea/urethane hybrid composition may be produced when a polyurea prepolymer is reacted with a hydroxyl-terminated curing agent. In another example, when a polyurethane prepolymer is reacted with amine-terminated curing agents during the chain-extending step, any excess isocyanate groups in the prepolymer will react with the amine groups in the curing agent. The resulting polyurethane composition contains urethane and urea linkages and may be referred to as a polyurethane/urea hybrid as discussed further below.
Golf ball covers made from polyurethane and polyurea compositions are generally known in the industry. In recent years, polyurethane and polyurea cover materials have become more popular, because they provide the golf ball covers with a desirable combination of “hard” and “soft” features. The relative hardness of the cover protects the ball from being cut, abraded, and otherwise damaged. In addition, such harder-covered golf balls generally reach a higher velocity when struck by a club. As a result, such golf balls tend to travel a greater distance, which is particularly important for driver shots off the tee. Meanwhile, the relative softness of the cover provides the player with a better “feel” when he/she strikes the ball with the club face. The player senses more control over the ball as the club face makes impact. Such softer-covered balls tend to have better playability. The softer cover allows players to place a spin on the ball and better control its flight pattern. This is particularly important for approach shots near the green. Polyurethane and polyurea covered golf balls are described in the patent literature, for example, U.S. Pat. Nos. 5,334,673; 5,484,870; 6,476,176; 6,506,851; 6,867,279; 6,958,379; 6,960,630; 6,964,621; 7,041,769; 7,105,623; 7,131,915; and 7,186,777.
As discussed above, isocyanates with two or more functional groups are essential components in producing polyurethane and polyurea polymers. These isocyanate materials can be referred to as multi-functional isocyanates. Such isocyanates can be referred to as monomers or monomeric units, because they can be polymerized to produce polymeric isocyanates containing two or more monomeric isocyanate repeat units.
Aromatic isocyanates are normally used for several reasons including their high reactivity and cost benefits. Examples of conventional aromatic isocyanates include, but are not limited to, toluene 2,4-diisocyanate (TDI), toluene 2,6-diisocyanate (TDI), 4,4′-methylene diphenyl diisocyanate (MDI), 2,4′-methylene diphenyl diisocyanate (MDI), polymeric methylene diphenyl diisocyanate (PMDI), p-phenylene diisocyanate (PDI), m-phenylene diisocyanate (PDI), naphthalene 1,5-diisocynate (NDI), naphthalene 2,4-diisocyanate (NDI), p-xylene diisocyanate (XDI), and homopolymers and copolymers thereof. The aromatic isocyanates are able to react with the hydroxyl or amine compounds and form a durable and tough polymer having a high melting point. The resulting polyurethane or polyurea material generally has good mechanical strength and cut/shear resistance.
However, one disadvantage with using aromatic isocyanates is the polymeric reaction product tends to have poor light stability and may discolor upon exposure to light, particularly ultraviolet (UV) light. Because aromatic isocyanates are used as a reactant, some aromatic structures may be found in the reaction product. UV light rays can cause quinoidation of the benzene rings resulting in yellow discoloration. Hence, UV light stabilizers are commonly added to the formulation, but the covers may still discolor or develop a yellowish appearance over prolonged exposure to sunlight. Thus, golf balls are normally painted with a white paint or other desirable color and then covered with a transparent coating to protect the ball's appearance.
In a second approach, aliphatic isocyanates are used to form the prepolymer. Examples of aliphatic isocyanates include, but are not limited to, isophorone diisocyanate (IPDI), 1,6-hexamethylene diisocyanate (HDI), 4,4′-dicyclohexylmethane diisocyanate (“H12 MDI”), and homopolymers and copolymers thereof. These aliphatic isocyanates can provide polyurethane and polyurea materials having generally good light stability. However, such polymers tend to have reduced mechanical strength and cut/shear-resistance.
As discussed above, golf ball covers having good light stability are needed. One objective of this invention is to develop a golf ball incorporating a cover having good light stability and meanwhile not sacrificing important mechanical properties such as shear-resistance/durability. Accordingly, it would be beneficial to develop polyurethane and/or polyurea compositions possessing such desirable properties. The present invention addresses and solves these needs.